This invention relates to a process for welding plastic vials and closures suitable for sterile filling operations. The process of the invention is suitable for any containers which must be filled and then sealed to maintain the contents in a leakproof enclosure, package or device.
Containers filled with liquid or other substances are often closed with a cap, stopper or other type of closure which uses mechanical means to effect the seal. When the closure uses an elastomer, such as rubber, as one of the sealing surfaces, bearing against a relatively hard surface, such as rigid plastic, glass or metal, a leakproof seal is generally obtained.
However, when a seal is created by mechanical means between two rigid materials, the surface must be nearly perfect, and free of any defects which will interfere with the seal. Alternatively, the assembly forces (usually torque) must be great enough to create a localized high pressure area, and thus some deformation of the interface to create the seal. This will often result in high assembly stress. Also, high assembly torque often lead to problems such as opening difficulty and sometimes stress fracture. Because of the difficulty in obtaining a leakproof joint with only mechanical means, heat seals or welds are often used between plastic containers and closures.
The fusion welding of thermoplastic materials generally requires pressure and temperature. The pressure must be at the interface between the materials being welded. The temperature must be provided by means for heating the interface above the melting temperature of the materials being welded. Examples of this type of welding include the induction seal on containers of liquid food products, solvents, lubricants, medications, etc. In these types of seals, a foil/plastic film laminate is heat sealed or welded to the container by heating at the interface while maintaining mechanical pressure. An example of this is a foil/plastic seal, where the seal is held against the plastic container, for instance, with a screw cap. The heat is applied directly to the interface by heating the metal first, which is laminated to the plastic film.
Thin materials, such as plastic films, which are about 0.25 mm thick, can be easily sealed by applying heat and pressure from heated platens on one or both sides of the joint. However, when the plastic is thick, e.g., 1.0 mm or thicker, it is difficult to transmit the heat through the plastic to the interface without distorting or thinning the plastic. A common solution to this is hot plate welding which places a heated metal platen between the two surfaces to be welded, which creates melting at the interface. The platen is removed and the surfaces are pushed together and held until solidified into a good weld. This process, while satisfactory in some situations, is not suited for sterile filled products with very crucial clean-room requirements such as pharmaceuticals. Other possible ways of welding thick plastics include vibration and ultrasonic welding but these also tend to be too prone to particulate generation for use in clean-room and sterile filling applications.
The process of this invention relates to a method of first creating a mechanical seal between a vial and closure plug comprised of the same, similar, or compatible thermoplastic resin material. A heated die of a specific shape is then used to remelt and reform the interface in order to weld the components into a single hermetically sealed container. The die generally has a concave shape in order to move the molten plastic resin into a confined volume. As the die continues its travel into the interface between the two components being joined, the confined volume becomes filled, and molten mass is now subjected to hydraulic pressure. The process is completed just after the confined area is completely filled and excess molten plastic begins to flow out of the joint. This excess material, frequently referred to as flash, is visual evidence of sufficient heat and pressure necessary to create a high quality weld.